Zafirlukast proven to be clinically safe and effective in the treatment of mild to moderate asthma. Patients having asthma exacerbations with FEV1 (spirometry <=70%) upon arrival in ED between ages 18-65 will receive B2-agonist, oral Prednisone, and either oral Zafirlukast 20 mg, 160 mg, or placebo administered in a predetermined randomized manner 30 minutes after baseline evaluation is completed and continued emergency therapy. Patients discharged home receive oral steroid, inhaled B2-agonists, asthma medications and either Zafirlukast 20 mg orally 2x/dayorplacebo (28 days).